Integrated circuits are typically supplied by one or more direct current (DC) power sources. Certain applications may require that an operating voltage which is higher or lower than the available supply voltages also be provided to the integrated circuit. For example, memory devices such as FLASH electrically erasable programmable read-only memories (EEPROMs) typically require such an operating voltage to perform read and write operations. Another example is a digital wristwatch, which needs a high voltage in order to drive the segments in its output display. Yet, in many instances it is not feasible to add an additional power source to provide the higher voltage.
An alternative to providing an additional, external power source is to use a charge pump circuit to generate a local-use supply voltage. A typical charge pump circuit includes one or more pumping capacitors that are selectively charged and discharged through a series of switches to boost the supply voltage and thereby provide the higher reference voltage. The switches are often driven by control signals having non-overlapping phases that are provided by an oscillator. Furthermore, the number of stages of switches and capacitors may be increased or cascaded to correspondingly increase the voltage boost by a desired multiple.
One example of a charge pump circuit providing a multiple voltage boost is U.S. Pat. No. 5,635,776 to Imi entitled "Charge Pump Voltage Converter." This charge pump circuit can generate a 4.times. or an 8.times. voltage boost and includes two pumping capacitors that are alternatingly controlled by a series of switches. The switches are each driven by respective phase signals that are generated using an oscillator and a frequency divider. While such a configuration does provide a significant voltage boost, it may not be feasible in applications where very little supply current is available because too much supply current may be consumed during the generation of the switch control signals. For the same reason, other prior art charge pump circuits that use multiple oscillators or other clock sources to provide the timing signals may also be inadequate where low current consumption is desirable, such as in portable battery powered devices.